"Cerberus the porter of Hell"

Creative Commons Attribution Share AlikeCerberus - Credit: Giuseppe Canino
Cerberus in Greek and Roman mythology is a terrifying, multi-headed hound, who guards the gates of the Underworld, or Hades, to prevent those who have crossed the river Styx from returning.  He is usually depicted with three heads, although this varies. 

Mythology has him as the offspring of Echidna, a half-woman half-serpent, and Typhon, a fire-breathing giant.